Interference competition for nectar sources has been repeatedly described between hummingbirds and various insects, but rarely recorded in other nectarivorous birds. We observed aggressive behaviour by African sunbirds (Cinnyris reichenowi and Cinnyris bouvieri) defending the nectar plant Hypoestes aristata against carpenter bees (Xylocopa caffra and Xylocopa inconstans) in the Bamenda Highlands, Cameroon. During 200 hours of observation, we recorded 38 cases of the sunbirds attacking carpenter bees; all these intrusions occurred only in the flower-richest patches of the plant.We predict that similar aggressive interactions will occur between other Old World nectarivorous birds and insects. Such interference competition between distantly related taxa could have an important impact on the evolution of pollination systems.